Since the discovery of carbon nanotubes, numerous preparation methods and applications of these materials have been realized. In recent times, there has been an increasing attention paid to nanotubes and nanofibers of greater chemical complexity, especially oxide nanotubes and nanofibers, and researchers have successfully prepared nanotubes of Al2O3, SiO2, V2O5, WO3, ZnO, ZrO2, and TiO2. Of these, nanotubes and nanofibers of titanium oxide-based materials are of particular interest because of the various interesting properties, such as photocatalytic, semiconducting, gas sensing, and like properties, which are observed in bulk TiO2.
For example, the excellent lithium intercalation behavior of these fibers exceeds that of currently available materials and they therefore may be advantageously used in high performance lithium ion batteries, e.g. for high performance and demanding applications, such as in hybrid automobiles. Highly efficient dye sensitized solar cells using single crystalline titania nanotubes have also been reported. These nanotubes acts as a thin film semiconductor with higher electron transfer through the tubes as compared to nanocrystalline TiO2 films. Use of these materials in high performance ceramic membranes has been reported. In addition, when doped with materials with such specific functionality, these materials can be used for other novel applications. The titanate nanotubes (or TiO2 nanotubes) are usually multi-walled, with dimensions can vary over a large range. These properties can be enhanced and new functionality developed when these materials are prepared in the form of nanotubes/fibers, and a continuing and unmet need exists for improved methods of making such materials.
The synthesis of titanate nanostructures using techniques known in the art typically takes from 24-72 hours. See, e.g. Jang, et al., “Synthesis of Sn-Porphyrin-Intercalated Trititanate Nanofibers: Optoelectronic Properties and Photocatalytic Activities,” Chem. Mater. 19(8), 1984-91 (2007); Jiang, et al., “Syntheses, Characterization and Properties of Novel Nanostructures Consisting of Ni/Titanate and Ni/Titania,” Materials Letters 60(29-30), 3803-08 (2006); Zhang, et al., “Formation Mechanism of H2Ti3O7 Nanotubes,” Phys. Rev. Lett. 91(25), 256103 (2003); Zhang, et al., “Electrochemical Lithium Storage of Titanate and Titania Nanotubes and Nanorods,” J. Phys. Chem. C, 111(16), 6143-48 (2007); Qamar, et al., “Effect of Post Treatments on The structure and Thermal Stability of Titanate Nanotubes,” Nanotechnology (24), 5922 (2006); Suzuki, et al., “Lithium Intercalation Properties of Reassembled Titanate/Carbon Composites,” J. Electrochem. Soc. 154(5), A438-43 (2007); Wu, et al., “Sequence of Events for The Formation of Titanate Nanotubes, Nanofibers, Nanowires, and Nanobelts,” Chem. Mater. 18(2), 547-53 (2006); Wu, “Co-Doped Titanate Nanotubes,” Appl. Phys. Lett. 87(11), 112501-03 (2005); Pavasupree, et al., “Synthesis of Titanate, TiO2 (B), and Anatase TiO2 Nanofibers from Natural Rutile Sand,” J. Solid State Chem. 178(10), 3110-16 (2005); Du, et al., “Preparation and Structure Analysis of Titanium Oxide Nanotubes,” Appl. Phys. Lett. 79(22), 3702-04 (2001); Menzel, et al., “Impact of Hydrothermal Processing Conditions on High Aspect Ratio Titanate Nanostructures,” Chem. Mater. 18(25), 6059-68 (2006); Ding, et al., “Preparation and Characterization of Fe-Incorporated Titanate Nanotubes,” Nanotechnology (21), 5423 (2006); Jitputti, et al., “Synthesis of TiO2 Nanotubes and Its Photocatalytic Activity for H2 Evolution,” Japanese J. Appl. Phys. 47(1), 751-56 (2008); Zhu, et al., “Phase Transition between Nanostructures of Titanate and Titanium Dioxides via Simple Wet-Chemical Reactions,” J. Amer. Chem. Soc. 127(18), 6730-36 (2005); Kolen'ko, et al., “Hydrothermal Synthesis and Characterization of Nanorods of Various Titanates and Titanium Dioxide,” J. Phys. Chem. B 110(9), 4030-38 (2006); Ma, “Nanotubes of Lepidocrocite Titanates,” Chem. Phys. Lett. 380(5-6), 577-82 (2003). In each of the foregoing references, a synthesis of a titanate nanostructure is described. In every case, the reaction times range from 10 hours to as high as 72 hours, with 24 and 48 hours being typical. Clearly such lengthy reaction times are impractical, and need exists for faster synthesis methods having shorter reaction times.